Various types of valves used in the oil and gas industry are operated by actuators. Chokes are a particular type of valve and are generally used as throttling devices to reduce downstream pressures or flow rates or regulate them to a predetermined set point or range of pressures or flow rates. Normally, other types of valves are in the system in which the choke is installed. Occasions can arise where it is necessary to obtain rapid closure of the particular system in which the choke is installed. Chokes have traditionally been operated by stepping actuators which allow for fine adjustment on the degree of opening of the chokes for control of the flowing liquid or gas stream. When a rapid shutdown has been required in the past, the stepping actuator was actuated in reverse. The problem with that mode of operation is that it took too long to close the choke, thus allowing the possibility of erosion damage to valves further downstream, which could see the effects of increased velocity until the choke could be closed fully, using the stepping actuator.
Yet other techniques would be to use hydraulically actuated motors and disable the stepping actuator drive mechanism and drive the stem under power of a hydraulic motor. The problem with these designs has been that the motor torque output of a hydraulic motor has to be carefully controlled or else it would apply potentially significant torques which could twist the stem completely, making the choke inoperable for further operations. Another shortcoming of using the hydraulic motor to actuate the stem for a rapid shutdown of the choke would be that hydraulic motors require high pressures and volume in the hydraulic pressure source. This entails the adaptation of a complex hydraulic system in order to make such a design functional for each individual actuator-operated choke.
One of the objectives of the present invention is to provide a simple mechanism which can rapidly close the choke when the situation warrants, yet still have the positional accuracy afforded by a stepping actuator. Another objective is to allow the choke to resume its old position at the time it was actuated for rapid closure after the situation requiring the closure has been corrected. This would, in turn, allow the system to stabilize because the old settings could be easily resumed. Yet another objective of the present invention is to provide the rapid closure function with a hydraulic system that employs high pressure but low volume, thus giving the potential for using seawater for the fluid medium for accomplishing rapid closure. Yet another objective of the present invention is to allow for rapid actuation of the actuator to close the choke, using the hydraulically operated assist, while at the same time allowing the choke to reopen at a more gradual rate to prevent damage to downstream equipment. These and other features of the present invention will be more readily appreciated by those skilled in the art from a review of the preferred and an alternative embodiment described in detail below.